[SPv2] Decomposite otherwise-compositable animations that paint nothing.

third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h

Index: third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h
diff --git a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h
index 688e3c2949416ae1d74ae808da00b17d78554253..553afd0523674f5cdd74e8e4be86fdd54c44e4b6 100644
--- a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h
+++ b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.h
@@ -49,13 +49,18 @@ class PLATFORM_EXPORT PaintArtifactCompositor {
   }
 
   // Updates the layer tree to match the provided paint artifact.
+  //
+  // Populates |compositedElementIds| with the CompositorElementId of all
+  // animations for which we saw a paint chunk and created a layer.
+  //
   // If |storeDebugInfo| is true, stores detailed debugging information in
   // the layers that will be output as part of a call to layersAsJSON
   // (if LayerTreeIncludesDebugInfo is specified).
   void update(
       const PaintArtifact&,
       RasterInvalidationTrackingMap<const PaintChunk>* paintChunkInvalidations,
-      bool storeDebugInfo);
+      bool storeDebugInfo,
+      CompositorElementIdSet& compositedElementIds);
 
   // The root layer of the tree managed by this object.
   cc::Layer* rootLayer() const { return m_rootLayer.get(); }